Recently, there has been significant interest in organic light emitting diodes (OLED's). These structures have begun entering the flat panel display market. A variety of consumer electronics products, such as car stereos and cell phones, are also available with OLED-based displays. In addition to these uses, a number of other applications, such as large-area signs and panels for general lighting applications, are envisioned for OLED's. However, a number of issues remain before OLED-based displays are able to reach their full potential. For example, as the area of an OLED increases, the probability of an electrical short occurring within the diode structure also increases. In a passive matrix architecture, a short may cause all pixels that share the same row and column electrodes to stay dark, thus potentially dramatically decreasing the brightness of an entire panel. For a flat panel display, the loss of a large number of pixels due to a short in one OLED may lead to unacceptable image degradation. The active matrix architecture does not suffer from this shortfall; however, it can be expensive and difficult to implement this architecture over arbitrarily large areas. Therefore, alternative architectures that allow for the monolithic fabrication of large area OLED panels, have tolerance to short circuits, and are compatible with low-cost manufacturing techniques (such as roll to roll) are desired.
A. R. Duggal, D. F. Foust, W. F. Nealon, and C. M. Heller at GE described an exemplary fault-tolerant architecture in an article published in Applied Physics Letters (vol. 82, pg. 2580 (2003)). They applied this exemplary architecture to the fabrication of an illumination panel. The panel consisted of individual OLED's that were placed side by side and were connected in series. This series connection was achieved by selectively removing the organic semiconductor from one edge of each device, thereby exposing the underlying anode electrode. The cathode of one device could then be electrically connected to the anode of its neighbor. Panels that exhibited fault tolerance and scalability were demonstrated. However, this architecture requires patterning of the organic layer, which increases the complexity of the manufacturing process and may lead to additional faults. Duggal et al. used laser ablation to pattern the organic layer. This step adds significantly to the complexity and might be detrimental to the speed of display manufacture.
Another alternative architecture for OLED's was proposed by J. Kido, T. Matsumoto, T. Nakada, J. Endo, M. Mori, N. Kawamura, and A. Yokoi, in their article HIGH EFFICIENCY ORGANIC EL DEVICES HAVING CHARGE GENERATION LAYERS, SID Digest, p. 964 (2003). This article focused on increasing the quantum efficiency of OLED's. Kido et al. described a cascaded OLED architecture where OLED devices were stacked on top of each other. Panels were built by repeating a basic unit that consisted of a metal, a p-type doped organic, an intrinsic organic, an n-type doped organic, and a metal. Repeating this structure twice gave two cascaded devices, in which the middle metal layer was injecting electrons in the n-type organic on one side, and holes in the p-type organic on the other. As a result of the doping, the middle electrode was able to serve both as an efficient anode, and as an efficient cathode. The cascaded configuration resulted in radiant flux and quantum efficiency that increased with the number of devices. Since the devices where stacked on top of each other, the radiant flux per unit area increased with the number of devices, leading to extremely bright panels. However, this architecture was specifically designed for increasing the efficiency of OLED's and was not meant to provide a fault tolerant way of building large area panels.
The present invention involves improved architectures for producing light emitting devices based on electroluminescence. These improved architectures may lead to simplified manufacture and allow for increased optical output, as well as increased tolerance to shorts.